This invention relates to non-destructive test apparatus for the inspection of annular welds using eddy current techniques.
The use of eddy current techniques for examining materials is well known; flaws in a material affect electrical resistivity and magnetic permeability and are thereby readily detected by variations in induced eddy currents. One technique, for example, as disclosed in British Pat. No. 1058862, comprises comparing the signals derived from a pair of spaced eddy current sensors arranged to interrogate the material and displaying the flaw indicating differential signal on an oscilloscope.
It has recently been proposed to use the technique for the inspection of annular tube/tube sheet welds of a tube-in-shell heat exchanger, the apparatus comprising a rotatable eddy current probe assembly having a pair of sensors with means for rotating the sensors about a weld bead and means for displaying the differential of the signals produced by the sensors, the differential being produced by means of a balanced bridge. When both the sensors are interrogating normal weld metal the random variation of the strength of the differential signal caused by multi-metal composition and rough surface tends to average out towards a mean balance so that an oscilloscope display of an acceptable weld presents a randomly oriented trace about a central point. A flaw or defect in the weld bead will result in a deviation in the trace from the central point and appears as a spike or finger extending from the central area of the display.
The spike or finger will indicate the location of the flaw in the weld bead and this location will be determined by reference to the position of the appropriate sensor. For a good weld the trace on the display can resemble a jumble or tangle of string, the complex nature of the trace being due to migration of the beam spot under the influece of differing amplitude and phase information as the probe interrogates different parts of the weld bead. The interpretation of such a trace is very difficult and it is an object of the present invention to provide apparatus which will give a pictorial display representing an annular weld and upon which is superimposed the position and relative size of any defect present in the weld bead.